This invention pertains to methods for forming thin dielectric films. More specifically, the invention pertains to methods of depositing a conformal film of dielectric material with a high degree of surface smoothness particularly suited to high aspect ratio gap fill applications in semiconductor device fabrication.
Conformal, uniform dielectric films have many applications in semiconductor manufacturing. In the fabrication of sub-micron integrated circuits (ICs) several layers of dielectric film are deposited. Four such layers are shallow trench isolation (STI), premetal dielectric (PMD), inter-metal dielectric (IMD) and interlayer dielectric (ILD). All four of these layers require silicon dioxide films that fill features of various sizes and have uniform film thicknesses across the wafer.
In particular, it is often necessary in semiconductor processing to fill a high aspect ratio gap with insulating material. As device dimensions shrink and thermal budgets are reduced, void-free filling of high aspect ratio (AR) spaces (AR>3.0:1) becomes increasingly difficult due to limitations of existing deposition processes. The deposition of doped or undoped silicon dioxide assisted by high density plasma CVD, a directional (bottom-up) CVD process, is the method currently used for high aspect ratio (AR) gap-fill. Evolving semiconductor device designs and dramatically reduced feature sizes have resulted in several applications where HDP processes are challenged in filling the high aspect ratio structures (AR>7:1) using existing technology (see, for example, U.S. Pat. No. 6,030,881). For structures representative of the 65 nm and 45 nm technology nodes, engineering the gap-fill process becomes structure dependent, hence the process requires re-optimization, a task of considerable complexity, every time a new structure needs to be filled.
An alternative to CVD is atomic layer deposition (ALD). ALD methods involve self-limiting adsorption of reactant gases and can provide thin, conformal dielectric films within high aspect ratio features. The ALD process involves exposing a substrate to alternating doses of, usually two, reactant gasses. As an example, if reactants A and B are first and second reactant gases for an ALD process, after A is adsorbed onto the substrate surface to form a saturated layer, B is introduced and reacts only with adsorbed B. In this manner, a very thin and conformal film can be deposited. One drawback, however, to ALD is that the deposition rates are very low. Films produced by ALD are also very thin (i.e., about one monolayer); therefore, numerous ALD cycles must be repeated to adequately fill a gap feature. These processes are unacceptably slow in some applications in the manufacturing environment.
Another more recently developed technique useful in gap fill and other dielectric deposition applications in semiconductor processing is referred to as pulsed deposition layer (PDL) processing, sometimes also referred to as rapid surface-catalyzed vapor deposition (RVD). PDL is similar to ALD in that reactant gases are introduced alternately over the substrate surface, but in PLD the first reactant A acts as a catalyst, promoting the conversion of the second reactant B to a film. In ALD the reaction between A and B is approximately stoichiometric, meaning that a monolayer of A can only react with a similar amount of B before the film-forming reaction is complete. The catalytic nature of A in PDL allows a larger amount of B to be added, resulting in a thicker film. Thus, PDL methods allow for rapid film growth similar to using CVD methods but with the film conformality of ALD methods.
Current PDL-type processes involve the use of a metal or metalloid containing catalyst. As an example of the use of PDL to deposit silicon dioxide on silicon, the first reagent can be trimethylaluminum (TMA) and the second tirs(pentoxy)silanol (TPOSL). The heated silicon substrate is first exposed to a dose of TMA, which is thought to react with the silicon surface to form a thin layer of surface-bound aluminum complex. Excess TMA is pumped or flushed from the deposition chamber. A large dose of TPOSL is then introduced. The aluminum complex catalyzes the conversion of the silanol to silicon oxide until the silanol is consumed, or the growing film covers or otherwise inactivates the catalytic complex. When excess silanol is used, the film growth is usually self-limiting and a thick and uniform film results. Unreacted silanol may now be removed from the chamber and the growth cycle repeated. Other metal-containing precursors that can be deposited to activate or reactivate the catalytic surface include, but are not limited to, precursors containing zirconium, hafnium, gallium titanium, niobium, tantalum, and their oxides or nitrides.
In front-end of line (FEOL) applications the metal content of the dielectric films is very tightly controlled, and the presence of metals in concentrations of 1-2% atomic may not be desirable. The metal or metalloid compound results in the formation of second phase (e.g., Al2O3 in the case of an Al-containing precursor) in PDL silicon oxide, which can affect film properties. Moreover, prior work in the field indicates a phosphorus incorporation scheme that implies that phosphorus concentration in the film will be increasing proportionally to that of the metal or metalloid. Since P-doped silica films contain typically >5 wt. % P, similar concentrations of metals/metalloids will be incorporated into the film, which is likely to impact subsequent process steps (CMP, contact etch, etc).
Therefore, a method is needed to eliminate the presence of metals and metalloid elements from the silica films (both doped and undoped), and at the same time maintain conformality, growth rate, and other film properties.